On Tue, Jul 22, 2025 at 8:08 PM Andres Freund <and...@anarazel.de> wrote: > My response was specific to Tomas' comment that for many queries, which tend > to be more complicated than the toys we are using here, there will be CPU > costs in the query.
Got it. That makes sense. > cheaper query expensive query > simple readahead 8723.209 ms 10615.232 ms > complex readahead 5069.438 ms 8018.347 ms > > Obviously the CPU overhead in this example didn't completely eliminate the IO > bottleneck, but sure reduced the difference. That's a reasonable distinction, of course. > If your assumption is that real queries are more CPU intensive that the toy > stuff above, e.g. due to joins etc, you can see why the really attained IO > depth is lower. Right. Perhaps I was just repeating myself. Tomas seemed to be suggesting that cases where we'll actually get a decent and completely worthwhile improvement with the complex patch would be naturally rare, due in part to these effects with CPU overhead. I don't think that that's true at all. > Btw, something with the batching is off with the complex patch. I was > wondering why I was not seing 100% CPU usage while also not seeing very deep > queues - and I get deeper queues and better times with a lowered > INDEX_SCAN_MAX_BATCHES and worse with a higher one. I'm not at all surprised that there'd be bugs like that. I don't know about Tomas, but I've given almost no thought to INDEX_SCAN_MAX_BATCHES specifically just yet. -- Peter Geoghegan
